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논문 : Al 첨가 TWIP강에서의 지연파괴에 대한 변형유기 마르텐사이트 변태의 영향
김영우 ( Young Woo Kim ),강남현 ( Nam Hyun Kang ),박영도 ( Young Do Park ),최일동 ( Il Dong Choi ),김교성 ( Gyo Sung Kim ),김성규 ( Sung Kyu Kim ),조경목 ( Kyung Mox Cho ) 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속·재료학회지 Vol.46 No.12
For the advanced high strength steels (AHSS), high-manganese TWIP (twinning induced plasticity) steels exhibit high tensile strength (800-1000 MPa) and high elongation (50-60%). However, the TWIP steels need to be understood of delayed fracture following the cup drawing test. Among the factors to cause delayed fracture, i.e, martensite transformation, hydrogen embrittlement and residual stress, the effects of martensite transformation (γ→ε or γ→α`) were investigated on the delayed fracture phenomenon. Microstructural phase analysis was conducted for cold rolled (20, 60, 80% reduction ratio) steels and tensile deformed (20, 40, 60% strain) steels. For the Al-added TWIP steels, no martensite phase was found in the cold rolled and tensile deformed specimen. But, the TWIP steels with no Al addition indicated the martensite transformation. The cup drawing specimens showed the martensite transformation irrespective of the Aladdition to the TWIP steel. However, the TWIP steel with no Al exhibited the larger amount of martensite than the case of the TWIP steel with Al addition. For the reason, it was possible to conclude that the Al addition suppressed the martensite transformation in TWIP steels, therefore preventing the delayed fracture effectively. However, it was interesting to note that the mechanism of delayed fracture should be incorporated with hydrogen embrittlement and/or residual stress as well as the martensite transformation.
열간프레스성형에서의 변형 측정장치 개발 및 기계적 거동의 물성화
유동훈(Donghoon Yoo),석동윤(Dong-Yoon Seok),김돈건(Dongun Kim),안강환(Kanghwan Ahn),손현성(Hyun-Sung Son),김교성(Gyo-Sung Kim),정관수(Kwansoo Chung) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.10
As a way to improve the safety of automotive and to reduce the weight of vehicles, new forming technologies and advanced materials are in high demand in the automotive industry. However, the advanced strength steel has inferior formability and large springback. In order to overcome such drawbacks, the hot press forming process (HPF) has been being applied for forming of automotive sheet parts. In this work, new equipment was suggested to measure unlimited displacement range compared to previous one which was able to measure only up to 10mm displacement range. The external extensometer connected with grips by wire was applied to equipment so that total strain range was measured up to failure also in high temperature. And the finite element analysis was conducted to characterize the mechanical properties of the HPF steel. Finally, the flow curves were represented by utilizing the Johnson-Cook type equation both in uniform and post-uniform deformation regions.
Fe-3%Si 단결정 박판의 자기적 이방성에 대한 해석
김교성,허무영,박노진,정태원 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.11
In order to evaluate the anisotropic magnetic properties of steel sheets, analytical expressions based on the orientation distribution function of samples were proposed and compared with experimental data in a Fe-3%Si single crystal. The magnetic anisotropy was measured by means of vibrating sample magnetometer(VSM) and the orientation was determined by X-ray texture goniometer. Anisotropy of the magnetic properties is found to be dependent on the magnetization energy which varies as a function of the crystallographical direction. Applying the measured magnetic properties to the analytical expressions, the anisotropy constants of K₄= 36500 J/㎥ and K_6 = 1000 J/㎥ were calculated. The highest magnetic permeability and flux density were obtained in the easy magnetization direction of [100]. The orientation independent magnetization energy of the Fe-3%Si single crystal was 16330 J/㎥.
섀도우 마스크용 극저탄소강의 항복점 연신율에 미치는 탈탄 분위기의 영향
황순영,김교성 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.3
Yield point elongation(YP-EL) should be suppressed in shadow mask after decarburization annealing due to higher yield strength and non-uniform deformation. The effect of decarburization annealing atmosphere was examined to optimize the processing conditions. Laboratory experimental results revealed that YP-EL was decreased with higher dew point and annealing temperature. However, hydrogen partial pressure and gas flow rate did not show significant effect on YP-EL. Calculated diffusion rate of carbon showed that the decarburization was controlled by surface reaction. The YP-EL of shadow mask was varied according to the stacked position in the production line. This phenomenon was analyzed by thermodynamic calculation of equilibrium carbon content with CO+CO₂ gas, which could be formed by decarburization reaction.
조질압연 조건이 극저탄소강의 결정립 조대화에 미치는 영향
권오준,김교성,백승철 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.11
This study was performed to clarify the effect of temper rolling conditions on the formation of abnormally coarse grain(ACG) structure in extra-low carbon steel. Both laboratory and mill trial temper rollings were carried out with various rolling reductions, followed by annealing at temperatures ranging between 660℃ and 700℃ for 3 hours. Optical microstructure and texture were examined using annealed specimens. FEM analysis was also performed to compare the effects of roll diameter, friction, and tension. The ACG was observed when the rolling reduction was higher than a critical value, which was found to be decreased at higher annealing temperatures. The formation of the coarse grains started at the surface layer of the specimens. Crystal orientation was not changed with the development of ACG, but the intensity of texture was decreased. When the rolling reduction was 2%, the ACG could be obtained in the specimens rolled in the mill. However, the ACG was not observed in the laboratory. FEM analyses showed that the discrepency between the two rolling mills was caused by the differences in the roll diameter and the friction coefficient.
저탄소 열연강판의 소부경화 특성에 미치는 화학조성 및 권취온도의 영향
권오준,김교성 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.4
Laboratory hot rolling experiments were performed to develop bake hardenable hot-rolled strips. The effects of steel chemistry as well as coiling temperature(CT) were studied to find out the optimum processing condition. It was found that the ferrite grain size was decreased as increasing the carbon content up to about 120ppm. The carbon content more than 120ppm did not affect the ferrite grain size significantly. When the carbon content was high matrix cementites were observed at the CT's ranging between 400 and 500℃, but few matrix cementites were observed at the CT's lower or higher than the range. Matrix cementite was not formed either when the carbon content was low. This can be analyzed by the degree of supersaturation and diffusivity of carbon. It was observed that the major factor which determined the amount of bake hardening (BH) was the content of soluble C and N. When CT was 200℃, BH was increased with the carbon content. When CT was 700℃, BH showed a maximum point at C=50-100ppm. When CT was 500℃, BH was low and did not show a carbon content dependency. This was attributed to the fact that precipitation of matrix cementites lowered the solute carbon content. BH in AI-containing steels was very low when CT was 700℃. This was thought to be due to the formation of AlN which not only decreased the solute nitrogen content but also accelerated the cementite precipitation. It was also found that fine grain size showed higher BH at a given solute (C+N) content.
냉연 후 어닐링 처리된 극저탄소강의 기계적 성질에 미치는 열연조건의 영향
권오준,김교성,장래웅 대한금속재료학회(대한금속학회) 1990 대한금속·재료학회지 Vol.28 No.3
A laboratory investigation was carried out to study the effect of hot rolling conditions on the mechanical properties of cold rolled and continuously annealed ultra-low carbon steels stabilized with Titanium. The drawability(r ̄-values) of cold rolled and annealed steels was found to be very sensitive to the microstructure of hot bands. Higher r ̄-values were observed when the grain structure of the hot bands were more uniform and finer. The as-hot rolled grain structure was fine and uniform when FT was higher than Ar₃. If FT was lower than Ar₃, the grain structure was pancaked or abnormally coarse when the amount of deformation was small. However, when the amount of deformation was sufficiently large and CT was relatively high, the grain structure was uniform and fine by the recrystallization of ferrite. This led to the development of a new hot rolling process where the reheating temperature was adjusted to a lower level so that strips were finish rolled in the ferrite region.